1. Field of the Invention
The present invention related to a method for producing a silicon single crystal, wherein the silicon single crystal is grown by the Czochralski method (CZ method) with or without performing necking operation, and a silicon seed crystal.
2. Related Art
In the conventional production of silicon single crystals according to the CZ method, a silicon single crystal is used as a seed crystal, which is brought into contact with silicon melt and then slowly pulled while being rotated to grow a single crystal ingot. In such a method, after the seed crystal is brought into contact with the silicon melt, the so-called necking is performed to form a neck portion having a smaller diameter of around 3 mm to eliminate dislocation propagated from slip dislocations generated in the seed crystal in high density due to thermal shock, then the diameter of the crystal is increased to a predetermined diameter, and a dislocation-free silicon single crystal is pulled. The necking operation performed as described above is widely known as the Dash Necking method, and has commonly been used for pulling a silicon single crystal ingot by the CZ method.
That is, conventionally used seed crystals have, for example, a cylindrical or prismatic shape with a diameter or side length of about 8-20 mm, and have a cut-away portion or a notch for attaching the seed crystal to a seed holder, and a flat bottom surface in a tip end thereof, which is initially brought into contact with the silicon melt. In order to safely pull a single crystal ingot while withstanding the weight of heavy single crystal ingot, it is difficult to use a thickness of the seed crystal smaller than the range mentioned above, considering the strength of the material.
Because a seed crystal having such a shape as described above has a large heat capacity of the tip end which is brought into contact with the melt, a large temperature difference is suddenly generated in the crystal upon the contact with the melt, and thus slip dislocations are generated at a high density. Accordingly, the aforementioned necking operation becomes necessary in order to eliminate these dislocations to grow a single crystal.
However, under the circumstance described above, the necking operation must be performed to a minimum diameter of 3-5 mm in order to completely eliminate the dislocations even if the other necking conditions are selected variously. The mechanical strength obtained by such a diameter has become insufficient for supporting a single crystal ingot getting heavier with recent use of a larger diameter of the silicon single crystals, and thus a serious accident threatens to occur, for example, the single crystal ingot falls due to breakage of the neck portion of a small diameter.
To solve these problems, the applicants of this application have previously suggested such invention as disclosed in Japanese Patent Laid-Open Publication No. 5-139880, and Japanese Patent Application No. 8-87187 (Japanese Patent Laid-Open Publication No. 9-255485). These inventions relate to techniques employing a seed crystal whose tip end has wedge shape or a hollow to reduce as far as possible the slip dislocations generated upon the contact of the seed crystal with the silicon melt, and thereby allowing dislocation-free production even when a relatively large diameter of the neck portion is used to improve the mechanical strength.
Though these methods are expected to improve the mechanical strength of the neck portion to some extent because of the use of a large diameter of the neck portion, they still perform the necking operation as ever, and hence form a necking portion containing slip dislocations. For the pulling of recent single crystals whose length and diameter are increasingly getting longer and larger, for example, which have a weight of 150 kg or more, the mechanical strength of the neck portion obtained even in these methods may become insufficient, and therefore they cannot be considered ultimate solutions.
Therefore, the applicants of the present application previously developed a method for converting crystals into single crystals without forming a neck portion, which is the most problematic factor as for ensuring the mechanical strength, and filed a patent application therefor (Japanese Patent Application No. 9-17687). This method uses a seed crystal having a tip end in a sharp-pointed shape or truncated sharp-pointed shape, which tip end is brought into contact with the silicon melt as the seed crystal. First, the tip end of the seed crystal is carefully brought into contact with the silicon melt, the seed crystal was let down at a low rate to melt the tip end of the seed crystal until it gets a desired diameter, and then the seed crystal is slowly pulled upwardly to grow a silicon single crystal ingot of a desired diameter without performing necking operation.
According to this method, because the contact area when the tip end of the seed crystal is initially brought into contact with the silicon melt and heat capacity of the tip end are small, thermal shock or steep temperature gradient does not occur in the seed crystal, and hence the dislocations are not introduced. Then, by letting down the seed crystal at a low rate to melt it down until the tip end of the seed crystal gets a desired diameter, steep temperature gradient is prevented, and the slip dislocations are not introduced into the seed crystal also during the melting down process. Finally, a silicon single crystal ingot can be grown to a desired diameter by slowly pulling the seed crystal as it is with no need to perform the necking, because the seed crystal has the desired diameter, no dislocation, and sufficient strength.
As described above, while temperature holding or heating of seed crystals above the melt, shapes or methods for reducing thermal shock upon seeding and the like have been suggested as means for lowering the initial dislocation density for the conventional necking seeding method, the diameter of the neck portion, which has a certain upper limit, has become to be unable to follow the production of larger and heavier single crystal ingots. In addition, such conditions do not necessarily afford a high rate of success in making crystals dislocation-free.
Therefore, the dislocation-free seeding method without performing the necking operation, which can cope with the use of such a larger diameter and heavier weight as mentioned above, has been established.
However, it is the rate of success in making crystals dislocation-free that may be a difficulty in the dislocation-free seeding method. That is, according to this method, if dislocations are once introduced, the operation cannot be reattempted unless the seed crystal is changed. Therefore, it is particularly important to improve the rate of success in making crystals dislocation-free. In addition, even though the seeding is performed in a dislocation-free state in this method, slip dislocations may be generated when the seed crystal is left at a temperature around the melting point of silicon for a certain period of time after a predetermined length of the tapered tip end of the seed crystal is melted, or depending on the period requiring for starting the crystal growth, and such dislocations may further increase. Investigations of the cause of this phenomenon revealed that the control of the factors which had conventionally been controlled, for example, shape of the seed crystal, temperature holding time above the melt surface, melting down rate, single crystal growing rate and the like, is not sufficient for eliminating the phenomenon, and such control could not afford so high rate of success in making crystals dislocation-free and sufficient reproducibility.
Further, as shown in FIG. 6(b), a conventional seed crystal holder have a structure where a straight body 2 of a seed crystal 1 is inserted into a cylindrical member of the seed crystal holder body, and the seed crystal is fixed with a taper pin 16 inserted from the side face of the cylindrical member into a notch 15 of the seed crystal straight body 2. However, the contact area between the notch 15 and the taper pin 16 is small in this structure, and therefore stress is concentrated thereon, thereby increasing the possibility of breakage.
Moreover, because the seed crystal 1 having sharp-pointed tip end used for the dislocation-free seeding method without performing the necking operation, for example, one shown in FIG. 6(a), has a straight body 2 for providing the notch 15, the straight body provides additional heat capacity. Further, the straight body present in the seed crystal holder provides additional volume, and hence the volume of the seed crystal holder itself, i.e., its heat capacity becomes larger. This not only lowers the rate of temperature increase when the seed crystal is approached to the melt surface, but also makes the temperature gradient during the melting down of the seed crystal into the melt or the pulling operation larger, and therefore it provides a condition that dislocations are likely to occur, and generated dislocations are difficult to be eliminated.
The present invention has been completed to solve the aforementioned problems of the prior art, and its object is to provide a seed crystal which is hardly introduced with dislocations during the process of the CZ method, aiming at improving the rate of success in making crystals dislocation-free in the dislocation-free seeding method with or without performing necking operation using the seed crystal, thereby providing a method for producing a silicon single crystal capable of improving productivity of single crystal ingots having large diameter and heavy weight.
To solve the aforementioned problems, the present invention provides a silicon seed crystal which is composed of silicon single crystal and used for the Czochralski method, wherein oxygen concentration in the seed crystal is 15 ppma (JEIDA) or less.
When a silicon seed crystal having an oxygen concentration in the seed crystal of 15 ppma (JEIDA) or less as defined above is used, oxygen does not precipitate during, for example, contact with the melt and melting down therein of the seed crystal, and substantially no slip dislocation containing precipitated oxygen as a nucleus is generated. Therefore, the rate of success in making crystals dislocation-free is improved regardless of the use of necking, and thereby productivity and production yield of dislocation-free silicon single crystals are improved.
The aforementioned silicon seed crystal preferably has a shape having a sharp-pointed tip end, or a truncated sharp-pointed tip end.
Because a seed crystal having such a shape would have an extremely small heat capacity of its tip end, thermal shock is attenuated when the seed crystal is brought into contact with the melt, and hence generation of slip dislocations is reduced. In addition, it synergistically further improves the rate of success in making crystals dislocation-free together with the suppressed oxygen concentration.
The present invention also provides a silicon seed crystal which is used for the Czochralski method, wherein the silicon seed crystal does not have a straight body.
Because such a seed crystal which does not have a straight body is substantially composed only of a portion serving as a seed crystal, the volume of the seed crystal as a whole is remarkably decreased, and thus unnecessary heat capacity is also decreased. As a result, the total heat capacity of the seed crystal and the seed crystal holder also becomes small, and the rate of temperature increase is increased when the seed crystal is approached to the melt surface. Moreover, the temperature gradient can be made smaller during its melting down and pulling after the tip end of the seed crystal is brought into contact with the melt. Therefore, dislocations becomes less likely to be generated, and already generated dislocations become likely to disappear. Furthermore, improvement of the rate of temperature increase may shorten the operation time, and hence is expected to improve the productivity and the production yield.
The aforementioned seed crystal preferably has a body shape selected from the group consisting of shapes of cone, pyramid, truncated cone, truncated pyramid, combination of cone and truncated cone, combination of cone and truncated pyramid, combination of pyramid and truncated pyramid, and combination of pyramid and truncated cone.
As the seed crystal which does not have a straight body, various shapes can be exemplified as mentioned above. As for the advantageous effects thereof, for example, a seed crystal having a cone shape is held by a crystal seed holder on a part of its side surface near its bottom face, or all of its side surface, and therefore the load withstanding property of the seed crystal itself may be improved. Further, the absence of the straight body reduces the total volume and heat capacity of the seed crystal and the seed crystal holder, and accelerates the rate of temperature increase when the seed crystal is approached to the melt surface. Moreover, the temperature gradient can be made smaller during its melting down and pulling after the tip end of the seed crystal is brought into contact with the melt. Therefore, dislocations become less likely to be generated, and already generated dislocations become likely to disappear. It is clear that a seed crystal having one of the shapes mentioned above other than the cone shape can exert substantially the same advantageous effects as those provided by the cone shape.
A part or all of side surface of the aforementioned seed crystal is preferably formed with curved surface.
For example, when a seed crystal which has a tapered cone shape tip end having a straight ridgeline is melted down into a silicon melt from the tip end at a constant rate, the melting interface diameter of the seed crystal becomes larger in proportion to the lapsed time. On the other hand, by using a silicon seed crystal a part or all of which side surface is formed with curved surface, increase rate of the diameter along the ridgeline can be made smaller compared with that provided by the straight ridgeline in the region of the cone whose side surface is formed with curved surface, and therefore the thermal stress in a portion where the diameter of the melting interface is getting larger is greatly attenuated. Accordingly, the probability of slip dislocation generation is reduced, and the region where dislocations are more likely to be generated is shifted to the thicker side. The pulling can be started from a point in a dislocation-free state defined after such a regional shift. This improves the rate of success in making crystals dislocation-free, and can sufficiently meet the requirements for growing single crystals having a larger diameter and heavier weight.
The aforementioned seed crystal which does not have a straight body of the present invention preferably has an oxygen concentration in the seed crystal of 16 ppma (JEIDA) or less.
By using such a low oxygen concentration in the seed crystal, oxygen does not precipitate during the contact and the melting down of the seed crystal not having a straight body in the melt, and substantially no slip dislocation containing precipitated oxygen as a nucleus is generated. These effects is realized by the fact that the shape of the aforementioned seed crystal reduces the total heat capacity of the seed crystal and the seed crystal holder, and hence the seed crystal is maintained at a high temperature of the melt to a certain height from the liquidus-solidus interface, which makes the precipitation of oxygen difficult. As for a seed crystal not having a straight body, the advantage can be more effectively exhibited when the initial oxygen concentration in the seed crystal is 16 ppma or less.
The present invention also provides a method for producing a silicon single crystal by the Czochralski method comprising bringing a tip end of a seed crystal into contact with a silicon melt to melt the tip end of the seed crystal, performing necking operation, and growing a silicon single crystal, wherein any one of the aforementioned silicon seed crystals of the present invention is used as the seed crystal.
In the above method of the present invention for producing a silicon single crystal wherein the single crystal is grown with performing the necking operation, for example, a tip end of a seed crystal having a sharp-pointed tip end shape or truncated sharp-pointed tip end shape is brought into contact with a silicon melt and melted down therein to a predetermined position, then the necking operation is performed to eliminate dislocations, a cone portion is formed, and then a silicon single crystal is grown in an intended diameter. In the aforementioned method of the present invention, because the heat capacity of the seed crystal is small, dislocations are less likely to be generated. In addition, because of the low oxygen concentration in the seed crystal, i.e, 15 ppma or less, or 16 ppma or less as for the seed crystal not having a straight body, oxygen does not precipitate during the melting down of the seed crystal, and substantially no dislocation containing precipitated oxygen as a nucleus is generated. Therefore, dislocation-free production becomes possible even when a relatively large diameter of the neck portion is used. Accordingly, the rate of success in making crystals dislocation-free is improved, and thereby productivity and production yield of dislocation-free silicon single crystals are improved.
In this case, the shape of the seed crystal should not necessarily be in a shape having a sharp-pointed tip end, and the rate of success in making crystals dislocation-free can be improved by using a seed crystal having a low oxygen content as defined in the present invention even if a conventional seed crystal having a flat bottom face is used.
The present invention further provides a method for producing a silicon single crystal by the Czochralski method comprising bringing a tip end of a seed crystal into contact with a silicon melt to melt the tip end of the seed crystal, and growing a silicon single crystal without performing necking operation, wherein any one of the aforementioned silicon seed crystals of the present invention is used as the seed crystal.
In the aforementioned method of the present invention for producing a silicon single crystal wherein the single crystal is grown without performing necking operation, for example, a tip end of a seed crystal having a sharp-pointed tip end shape or truncated sharp-pointed tip end shape is brought into contact with a silicon melt and melted down therein to a predetermined position, then immediately a cone portion is formed without performing necking operation, and a silicon single crystal is grown in an intended diameter. In the aforementioned method of the present invention, because the heat capacity of the seed crystal is small, dislocations are less likely to be generated. In addition, because of the low oxygen concentration in the seed crystal, i.e, 15 ppma or less, or 16 ppma or less as for the seed crystal not having a straight body, oxygen does not precipitate during the melting down of the seed crystal, and substantially no dislocation containing precipitated oxygen as a nucleus is generated. Therefore, the rate of success in making crystals dislocation-free is improved, and growing of silicon single crystals having a larger diameter and heavier weight becomes possible.
According to the present invention, the rate of success in making crystals dislocation-free can be improved when silicon single crystal ingots are pulled by the Czochralski method regardless of use or unuse of the necking operation. This effect can be obtained with good reproducibility, and stably obtained for a long period of time. Therefore, the present invention can cope sufficiently with future use of a larger diameter, longer length, and heavier weight of single crystals, and can markedly improve the productivity and the production yield, and markedly reduce the production cost.